In addition to performing surgical operations on animal tissues by means of mechanical instruments such as scalpels or knives, surgery may also be performed by passing radio-frequency current through animal tissues. Ihere are essentially four main surgical operations that can be performed depending on the voltage levels and the amount of power applied to the tissue. These operations are typically designated as dessication, fulgeration, cutting and cutting with hemostasis. Often, dessication is referred to as coagulation and sometimes dessication and fulguration are designated collectively as coagulation.
The radio-frequency current used in the performance of electrosurgical operations is typically generated by means of a radio-frequency generator connected to a power amplifier. The output of the power amplifier is in turn connected to the tissue mass by means of two electrodes. Surgical operations are performed by means of an "active" electrode which introduces the radio-frequency current into the tissue mass. Since, as mentioned above, electrosurgical effects are primarily dependent on the power and voltage applied, the active electrode typically has a small cross-section to concentrate the power and limit the surgical effects to a small, controlled area. A return path from the tissue mass to the generator for the radio-frequency current is provided by a "passive" or "patient" plate which has a large area to prevent electrosurgical effects from taking place at the current return location. Alternatively, a pair of active electrodes may be used in a "bipolar" mode in which the electrosurgical effects are confined to the sample of tissue between the two electrodes.
A dessication operation is performed by holding the active electrode in firm contact with the tissue. Radio-frequency current passes from the electrode directly into the tissue to produce heating of the tissue by electrical resistance heating. The heating effect destroys the tissue cells and produces an area of necrosis which spreads radially from the point of contact between the electrode and the tissue. Due to the nature of the cell destruction, the necrosis is usually deep. The eschar produced during the operation is usually light in color and soft. In order to produce optimal results in a dessication operation, an electrosurgical generator must be capable of providing several amperes (peak current) of radio-frequency current to moist tissue which has an impedance of approximately 100 ohms. Although the radio-frequency peak current density is high, the power delivered to the tissue is relatively low because of the low tissue impedance. In addition, the dessication waveform may be interrupted to produce an overall low duty cycle which helps to reduce cutting effects. Therefore, although the peak current values are high, the RMS value of the current is low. During a dessication operation the moisture in the tissue cells is driven off at a controlled rate and as the moisture content in the tissue decreases its impedance increases. Therefore, in order to keep the power applied to the tissue at a low value and prevent a cutting effect, as described below, it is necessary to limit the power output as the tissue impedance increases. Ideally, the power decrease should be proportional to the impedance.
As the impedance of the tissue increases, depending on the output characteristics of the electrosurgical generator, another surgical effect can be produced. Cutting occurs when sufficient power per unit time is delivered to the tissue to vaporize cell moisture. If the power applied is high enough a sufficient amount of steam is generated to form a layer of steam between the active electrode and the tissue. When the steam layer forms, a "plasma" consisting of highly ionized air and water molecules forms between the electrode and the tissue causing the tissue impedance, as seen by the generator, to rise to approximately 1000 ohms. If the electrosurgical generator can provide sufficient power to a 1000-ohm load and has sufficiently high output voltage, a radio-frequency electrical arc develops in the plasma. When this happens the current entering the tissue is limited to an area equal to the cross-sectional area of the arc where it contacts the tissue and thus the power density becomes extremely high at this point. As a result of the locally high power density the cell water volatizes into steam instantaneously and disrupts the tissue architecture - literally blowing the cells apart. New steam is thereby produced to maintain the steam layer between the electrode and the tissue. If the power density delivered to the tissue mass is sufficient, enough cells are destroyed to cause a cutting action to take place. A repetitive voltage waveform, such as a sinusoid, delivers a continuous succession of arcs and produces a cut with very little necrosis and hemostasis.
It is also possible to achieve a combination of the above effects by varying the electrical waveform applied to the tissue. In particular, a combination of cutting and dessication (called cutting with hemostasis) can be produced by periodically interrupting the continuous sinusoidal voltage normally used to produce an electrosurgical cut. If the interruption is of sufficient duration, the ionized particles in the plasma located between the electrode and the tissue diffuse away, causing the plasma to collapse. When this happens the electrode comes in contact with the tissue momentarily until a new plasma layer is formed. During the time that the electrode is in contact with the tissue it dessicates the tissue thereby sealing off small blood vessels and other bleeders in the vicinity of the electrode.
Another surgical effect called fulguration may be obtained by varying the voltage and power per unit time applied by the electrosurgical generator. Although fulguration is often confused with dessication, it is a distinctly different operation. In particular, fulguration is typically performed with a waveform which has a high peak voltage but a low duty cycle. If an active electrode with this type of waveform is brought close to a tissue mass and if the peak voltage is sufficient to produce a radio-frequency arc (at an impedance of 5000 ohms before electrical breakdown), fulguration occurs at the point where the arc contacts the tissue. Due to the low duty cycle of the fulgurating waveform, the power per unit time applied to the tissue is low enough so that cutting effects due to explosive volatization of cell moisture are minimized. In effect, the radio-frequency arc coagulates the tissue in the immediate vicinity of the active electrode thereby allowing the operating surgeon to seal off blood vessels in the vicinity of the electrode. The fulgurating electrode never touches the surface of the tissue and a hard, dark eschar is formed at the surface of the tissue mass in the fulgurated area. In contrast to dessication, fulguration is a surface process and the area of necrosis is confined to the surface. Therefore, fulguration can be used where the tissue mass is very thin and the deep necrosis produced by a dessication operation would damage underlying organs and accordingly, is a very useful operation.
In order to perform the above four surgical operations properly, a general-purpose electrosurgical generator must be capable of delivering significant amounts of radio-frequency power into a tissue impedance which varies over an order of magnitude (between approximately 100 ohms to approximately 1000 ohms). In addition, the generator must be capable of producing a sufficient peak voltage to initiate sparking in the fulguration and cutting modes. These requirements necessitate that the generator be capable of handling high internal radio-frequency voltages and currents.
In order to meet the internal generator demands the earliest prior art generators used oscillators and power amplifiers comprised of electron tube circuits. These prior art units had a disadvantage in that they dissipated large amounts of heat internally. In order to handle the internal heat load the units were large and bulky and required ventilating fans which exhausted non-sterile air into the operating room environment.
To reduce the heat problem, subsequent prior art units used semiconductor components to generate the required radio-frequency power output. The semiconductor devices inherently dissipated less heat than the electron tube counterparts, but did not entirely eliminate the heat loading problem. When the semiconductor devices were used in a linear mode they still dissipated significant amounts of heat internally.
Other units utilized semiconductor switching circuits to produce rectangular waveforms instead of the sinusoidal waveforms used by the previous units. These rectangular waveforms could be generated more efficiently than sinusoidal waveforms but still did not entirely eliminate the heat problem. In particular, because the semiconductor devices in a practical general purpose generator are required to handle both high voltages and high currents, high power semiconductor switching devices were often used. These devices were able to handle the required voltages and currents, but had the disadvantage that their switching times were slow. A slow switching time results in high internal heat dissipation. Therefore, many prior art semiconductor devices still required large and bulky heat-sinks or ventilating fans. Although semiconductor components were available which had fast switching times and therefore low internal heat dissipation, these devices were not used in prior art general purpose electrosurgical generators because they were not inherently capable of handling the high voltages and high currents required. In addition, the use of a non-sinusoidal waveform produced significant amounts of radio-frequency noise due to the high order harmonics in the output signal.
Other prior art general purpose generators have attempted to overcome the internal heating problem by using several separate semiconductor generating circuits, each optimized for a particular electrosurgical operation. This prior art approach allows the semiconductor circuitry to be tailored to each output required for the associated electrosurgical operation. The tailoring reduces the current and voltage requirements placed on the semiconductors and thus semiconductors can be used which have faster switching times and thus less internal power dissipation. Unfortunately, the multiplicity of components necessary for this approach produces expensive and bulky units.
Still other units have solved the problem by optimizing the generator for one or two electrosurgical operations. These units are small and compact but typically produce poor results in surgical operations other than those for which they were designed.